1. Field of the Invention:
This invention is a gas processing method for separating and recovering carbon dioxide (CO2) from gas streams having a high carbon dioxide content (e.g., 85-95 mol percent) and a low methane and/or nitrogen content. The novel process is particularly useful in processing gas streams produced during CO2 flooding operations for enhanced oil recovery.
2. Technology Review:
Various reservoir flooding techniques have been utilized by the oil and gas industry in enhanced oil recovery programs as a means to increase the production of hydrocarbons.
Carbon dioxide flooding is one technique that has been effectively used in the past, and is likely to become increasingly important in the future. In such operations, carbon dioxide is pumped into the reservoir through an injection well for extended periods of time (e.g., years). The injected carbon dioxide "floods" the treated zone and forces/carries the oil in the formation toward one or more production wells where the fluids are recovered. The composition of the produced fluids changes with time and, at some point, carbon dioxide "breakthrough" will occur. After breakthrough the volume of gas and the carbon dioxide content of the produced fluids increase substantially. Carbon dioxide may represent 60-96 mol percent (or more) of the fluids produced. In order for carbon dioxide flooding operations to be economically viable, carbon dioxide must be efficiently recovered from the produced fluids for reuse. In many cases, recovered carbon dioxide can be reinjected into the formation through the injection well, provided chemical specifications for purity are met. Product specifications for carbon dioxide can be quite high, particularly with respect to the content of hydrocarbons (i.e., methane and ethane) and/or nitrogen.
Various gas processing methods have been used to separate and recover carbon dioxide as well as the hydrocarbon/nitrogen components from gas streams having a high carbon dioxide content. The method (and equipment) chosen has depended, at least in part, on the product specifications for the carbon dioxide product.
Generally, where product specifications are high, current technology separates nitrogen and methane from carbon dioxide in one of three general ways:
(1) preferential passage of carbon dioxide through a semi-permeable (differentially permeable) membrane;
(2) absorption of carbon dioxide by a chemical or physical solvent, followed by regeneration of the solvent; and
(3) distillative or extractive distillative processes. The latter type of process is illustrated, for example, by the so-called "Ryan/Holmes" technology. Further information on the Ryan/Holmes technology is found in the paper presented under the title "The Ryan/Holmes Technology and Economical Route for CO2 and Liquids Recovery," presented by Ryan et al. at the AIChE winter meeting in Atlanta, Georgia on Mar. 11-14, 1984. Reference is also made to U.S. Pat. Nos. 4,293,322, 4,318,723, 4,383,842 and 4,428,759 which relate to one or more aspects of the Ryan/Holmes technique.